Transcriptional programs are controlled by the interplay between DNA sequence, transcription factors, and epigenetic mechanisms. We report genome-wide characterization of GATA3 binding sites in eleven well-defined developmental and effector cell types of the T lymphocyte lineage. By utilizing a conditional allele of Gata3, we investigated the impact of GATA3 expression on the mRNA expression patterns in several of these cell types. Correlation of GATA3 binding with gene expression changes indicates that GATA3 regulates a large number of stage- and cell-specific genes involved in multiple signaling and transcriptional pathways critical for T cell differentiation and immune responses. In addition to a primary GATA motif, GATA3 recruitment to a significant fraction of target sites may be mediated by secondary motifs depending on cellular context, suggesting a mechanism of expanding the regulatory complexity of a limited number of sequence-specific transcription factors. We show that GATA3 mediates H3K27me3 and H3K4 methylation to facilitate transcriptional repression or activation, respectively.